Mulches and Row Covers

One of the most important changes in the materials and methods of gardening over the last twenty years is the development of agricultural plastics. Nonchemical methods of controlling weeds and pests, inexpensive ways to lengthen the growing season, and water-stingy drip irrigation: all have been made possible by plastics technology.
     Mulch is an essential part of many gardens, particularly organic gardens; the range of materials used as mulch rivals that used for building a compost pile. I have seen just about everything from flat stones and boards, to straw, hay, seaweed, old pieces of carpet, and even thick layers of newspaper used as a mulch. Think about the nature of what you use. Newspaper, for example: black-and-white newsprint is generally considered safe, though for aesthetic reasons I wouldn’t use it in my garden; and if the paper uses colored inks, they may contain significant amounts of toxic compounds. In fact, I use very little mulch at all, as most materials that work well as a mulch are also attractive to slugs, one of the most obnoxious pests of humid-climate gardens.
     Plastic mulches made from very thin polyethylene (only a few thousandths of an inch thick) provide many of the benefits of an organic mulch without giving slugs a place to hide. Over the course of the season polyethylene mulches can prevent weed growth, preserve soil moisture, and raise or lower the temperature of the top few inches of soil. The most high-tech kinds are wavelength-selective. Infrared transparent (IRT) mulch, for example, allows the heating wavelengths of sunlight through, but blocks those wave lengths that drive photosynthesis, thus preventing weed growth under the mulch. Growers in hot climates can choose mulches that prevent overheating of the soil, and even more specific control is possible (see sidebar).
     The only benefit of an organic mulch that plastic can’t provide is the addition of organic matter to the soil. In fact, while polyethylene becomes brittle and shreds easily after only a few months in the sun, the resulting scraps of plastic are nearly indestructible and present a real disposal problem. Newer formulas for making the plastic include additives to make the mulch degrade further, but its essential nature is unchanged. Plastic mulches are inexpensive to buy and come in three- or four-foot widths, just right for raised beds, but season-end disposal of the used plastic brings home its environmental cost. As with other industrial materials, I recommend using plastic mulch only when absolutely necessary, and with full knowledge of its true life-cycle costs.
     There are film mulches made from recycled fiber, treated with just enough preservative to retard their breakdown until late in the season, when the plants have grown over the row and need no help to shade out sprouting weeds. Unfortunately, they are relatively expensive to use, and the preservatives themselves are not beyond suspicion as contaminants of the soil. If you do use a film mulch—either kind—be sure to put irrigation lines beneath it, or lay it down only when the soil is at optimum moisture levels. Mulch laid on top of wet soil will keep it soggy throughout the season, and if laid on dry soil, these largely waterproof films will nearly starve the plants for water.
     Another recent use for plastic is in covers that can be used to rig “mini-greenhouses” or cloches (plant covers) over growing plants. There are two basic types; conventional clear films and textured, milky white “spunbonded” fabric. Within each type there are a number of variations in material.
     Clear plastics, because of their tendency to overheat in sunny weather, must be somehow supported above the plants and vented. Irrigation also needs to be considered, since most clear covers shed rain. The most common arrangement is a very thin, clear cover about a foot or two tall and two feet wide, with parallel longitudinal slits cut in the plastic for ventilation, supported by hoops made from 9-gauge fence wire stuck six to eight inches into the ground on each side of the row. A plastic soil mulch is almost required when using this kind of cover, because the environment under the cover favors weeds growth as much as it does the crop plants. All in all, the labor involved in erecting these cover-and-mulch combinations is barely worth the few weeks’ use they get in short- spring areas. However, in milder climates, or in areas where temperatures aren’t extreme but rarely hot—like most coastal climates, for instance—they can be a very effective way to provide a protected microclimate. Garden suppliers now offer a wide range of hoops and cover sizes as well, making it possible to accommodate all kinds of plants.
     Fabric covers don’t overheat as easily, and have a number of other advantages over clear covers. First off, they are so lightweight (the lightest weigh in at only a third of an ounce per square yard) that while they can be used with hoop supports, they don’t need it. These “floating” row covers are simply laid over top of the row, and the edges buried, leaving all the slack fabric loose above the plants. Multiple layers (or simply thicker fabrics) can provide significant frost protection.
     As the crops grow, they pick up the row cover like the foil top on one of those prepackaged pans of popping corn. Also, since they are porous instead of solid, they allow the passage of both air and water, eliminating the need to provide ventilation and irrigation. But because they accelerate the growth of weeds as well as crop plants, you’ll want to put down a film mulch, or periodically remove the cover to cultivate the bed beneath.
     Perhaps best of all, even the lightest floating row covers, if thoroughly sealed with soil around the edges, will keep out all kinds of flying insects. This function (as a pest barrier) alone more than offsets the fact that their manufacture is just as energy-intensive as that of plastic film mulches and row covers. Pest problems for which most gardeners would otherwise have to spray—with either a synthetic or an organic pesticide—can now be controlled with row covers. For example, a floating row cover placed over broccoli transplants, immediately after setting out, is more effective in preventing cabbage root maggot infestations than the insecticide diazinon. The same row cover will protect all kinds of plants from flea beetles, as well as keeping cucumber beetles from attaching squash family plants until they are large and vigorous enough to outgrow the attack.
     Keep in mind, however, that many garden vegetables need to be accessible to insects in order to be pollinated. This requires removing the cover once the plants flower (specific recommendations can be found in the individual vegetable entries in Chapter 8.
     Not only can row covers protect plants from pests and cold, but they can provide protection from the heat, too. Lath and metal screening, as well as cheesecloth, have long been used to provide shade for cool-weather crops when the temperature rises, but new woven plastic meshes are lighter, easier to install, and can be designed for just about any degree of shade. They are made of a much more substantial thickness of plastic, woven like burlap, and so will last for many seasons, though it still makes sense to store them in the dark during the off-season to decrease the rate at which sunlight makes them brittle.

Comments

[zazel loven]

Hi - would love to interview you for Organic Gardening magazine on how to build hoop house - thank you for your consideration.
Zazel Loven

[Lucy Goodman]

I would like to comment on using row covers without support. I have been using covers extensively for about 8 years and I have found most plants are very disturbed by the weight of even the lightest covers directly on them. The leaves tend to become deformed and growth slows down.

I also have found that burying the edges of the covers can be a bad idea because it makes the covers difficult to remove for weeding and harvesting. We use rocks (around 5 to 10 pounds) to keep our covers down, some people use staples.

I find it is good to check the crop under the covers at least weekly to make sure all is well. Otherwise you can get into the out of sight out of mind syndrome with covered crops.

I remove covers for weeding (especially mesclun mixes and carrots which I do not mulch) weekly. After weeding is done the cover is put back on ASAP. I also remove covers to harvest things like mesclun and replace when I am done.

lucy